Surface plasmon resonances in metal nanostructures can lead to novel optical properties. The greatly enhanced
electromagnetic field makes surface-enhanced Raman scattering (SERS) a highly sensitive spectroscopic technique. We
employed Ag nanowires as plasmonic waveguide and achieved
remote-excitation SERS at a few molecules level. The
junctions between metal nanowires and nanoparticles offer hot spots for SERS, while the enhancement strongly depends
on the laser polarization. We studied the polarization dependence in Au nanowire-nanoparticle systems of different
geometry. The polarization of Raman-scattered light in SERS is a rarely studied topic. We found nanoantennas
composed of a few nanoparticles can manipulate the polarization of emission light. A nanoparticle trimer is the simplest
nanoantenna to realize the polarization control. By tuning the position and size of the third particle, emission polarization
can be modified in a controllable way. In addition, the refractivity of the surrounding media also plays a crucial role for
the emission polarization.
The detection of trace quantities of aromatic compounds is important to defense and security
applications, including the detection of CB agents, explosives, and other substances. These pose
threats to forces and the environment. This paper explores an approach to the detection and
identification of quantities as little as single molecules of explosives. It can in principle provide
Apertureless near-field scanning optical microscopy (ANSOM) is one of several promising
methods for obtaining spatial resolution below the diffraction limit at various wavelengths,
including in the terahertz regime. By scattering incident light off the junction between a probe
with a sub-wavelength tip and the surface of a sample, spatial resolution on the order of the tip
size can be obtained. For terahertz time-domain spectroscopy where the wavelength-limited
resolution is ~1 millimeter, this is a significant advantage.
In the case of a sufficiently small probe tip and a thin metallic substrate, plasmonic interaction
between the tip and sample provides an enhancement of the near-field in the junction. This
effect is dramatically enhanced for nanometer-scale metal layers, since surface plasmon states
from both sides of the film can contribute to the overall field enhancement.
We present preliminary results of THz plasmonic field enhancements, using a thin (500 nm) gold
film evaporated on glass. We observe an enhancement in the scattered THz wave, which we
attribute to the large density of plasmonic states extending throughout the THz range. This result
indicates a route to single-molecule spectroscopy at terahertz frequencies.
Using the Plasmon Hybridization (PH) method and the Finite-Difference Time-Domain (FDTD) method, we investigate the plasmonic properties of finite metallic nanopartices interacting with extended metallic substrates such as thin films and wires. The results from the two computational methods are found to agree very well. We show that the plasmons of a metallic nanoparticle couple to the delocalized plasmons of extended substrates in the same manner as an electronic impurity level couples to an continuum of electronic states. The interaction can result in both localized plasmons and virtual states in the plasmonic continuum. The virtual states are composed of delocalized plasmons from the extended substrate and depend sensitively on the geometry of the system, the polarization of incident electromagnetic excitations as well as the background dielectric properties of the structures. We demonstrate that the virtual state can provide large electric field enhancements over a broad and tunable spectral regime. Our investigations show that plasmonic structures supporting virtual states are highly suitable as substrates for surface enhanced spectroscopic applications and may be useful in plasmonic waveguiding applications.